I agree with Owlkat that you pretty much have to stick with your natural singing voice. HOWEVER, much can be done with varying songs with different keys, tempos, subject matter, etc. One of the great things about "folk" music is its many facets. When entertaining you have to do just that....ENTERTAIN....not put people to sleep. Don't be over-concerned with your voice, per se. Push your range a little. Sing like you feel it...like you would in your car or in the shower.